Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011

While not an elevation map, the corneal thickness (pachymetric map) represents the spatial difference between the anterior and posterior corneal surfaces and as such is totally dependent on accurate elevation data. In addition to identifying thin corneas, the overall pachymetric distribution may be another indicator of pathology. Normal corneas are typically thinnest in the central region and thicken in the periphery. Displacement of the thinnest region is often seen in keratoconus and may at times predate changes on either the anterior or posterior surfaces (FIGURE 15 – SAMPLE Isolated Pach Displacement).

Figure 15. The 4-view composite map (Oculus Pentacam) above shows an asymptomatic patient presenting for refractive surgical evaluation. Although there are no obvious abnormalities in the curvature or elevation the large displacement of the thinnest point from the corneal apex may suggest early ectatic disease.

DISPLACED APEX SYNDROME

Previously we discussed some of the limitations of trying to use curvature to depict shape. Early studies in patients seeking refractive surgery reported an incidence of “form fruste” keratoconus or “keratoconus suspect” as high as 17% of apparent normal individuals.34 Certain investigators initially pointed out that this high false-positive rate was related to the limitations of sagittal or axial-based curvature reconstructions and Placido-derived topography

Figure 17. A four-image composite map of a normal astigmatic cornea (Oculus Pentacam). The asymmetric bowtie pattern seen in the curvature map is created when the reference axis and the corneal apex do not coincide. The anterior elevation map shows that the apex of the cornea is displaced superiorly but the elevation is normal.

The classic asymmetric inferior bowtie pattern can be produced by a completely normal astigmatic eye if the curvature’s reference axis does not go through the corneal apex

(SEE FIGURE 2). (In actuality what more typically occurs is that the patient does not look through the center of their cornea, the so called positive angle kappa). Patients with a displaced apex syndrome typically have normal pachymetry, orthogonal astigmatism, stable refractions, and BSCVA of 20/20 or better.1 Many patients in the literature who have been described as having early keratoconus based solely on curvature maps (and reported to have excellent results from refractive surgery) have instead what is more likely a “displaced apex syndrome” and would probably not meet the criteria for keratoconus on elevation topography.36-38

SUMMARY

Elevation based topography offers important advances over Placido based devices. The ability to image the posterior cornea and to produce an accurate pachymetric map is in itself significant. Elevation maps are also more accurate in determining the cone morphology and in separating the false positive keratoconus suspect often due to a displaced corneal apex.

REFERENCES

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This is not an uncommon approach. When one wants to highlight an abnormality, you typically attempt to remove the “background noise.” In the case of elevation data, the “background” noise is any shape that will help demonstrate the clinically significant abnormalities.

For refractive surgery screening and for most clinical situations using a best-fit-sphere gives the most useful qualitative map (i.e. easiest to read and understand). Fitting a best-fit- sphere to the central 8.0 mm zone appears best. Since the normal eye is an aspherical prolate surface the central 8.0 mm zone yields a reference surface that allows for subtle identification of both ectatic disorders and astigmatism. Larger zones would typically yield a flatter BFS and smaller zones a steeper BFS. Because the maps we call “elevation” are actually subtraction maps

Effects of BFS Diameter on the Appearance of the Elevation Map

Diameter = 11.94 mm

Figure 2. Three anterior elevation maps showing the effect of varying the area used to compute the BFS. The upper left sample uses an area of 7.0 mm, the upper right 9.0 mm and the bottom 11.94 mm. As the area gets larger it incorporates more peripheral (flatter portions of the cornea) data. A flatter reference surface will make the “normal” prolate cornea appear to have a central “island.” This is a good example of the importance in keeping the BFS computation area consistent.

(showing how the raw elevation data appears compared to the reference surface) the shape of the reference surface will greatly affect the appearance of the map. Since the normal eye is steeper centrally and flattens in the periphery a flatter reference surface (i.e. a very large optical zone) will accentuate the central steep zone and the eye will appear to have an “island” (FIGURE 2). Similarly, a smaller optical zone will be steeper and may mask subtle cones.4

The Pentacam has two settings (AUTO & MAN) that determine how the machine selects the area used for the BFS. An understanding of these settings is mandatory to utilize the full capability of the system. The AUTO (Automatic) setting looks for the largest circle around the apex that can be drawn without any extrapolated data. The machine then selects the area set at 90% of this size. This insures that only valid data is utilized. The advantage of the AUTO system is that only valid data points are used. The disadvantage is that the area used to compute the BFS is variable in size. This makes comparisons difficult and the development of normal values impossible. If the image was of very high quality (no extrapolated data > 10.0 mm) it was possible to have the area used to define the BFS greater than > 9.0 mm. As the zone gets larger and incorporates more peripheral data (peripheral cornea is flatter) the Best-Fit shape would be flatter. When the cornea is then compared to a flatter reference surface, you can see positive islands of elevation that would normally be considered abnormal when compared to a reference surface computed form a smaller optical zone (e.g. 8.0 mm). Similarly, if a smaller optical zone was chosen the BFS would be steeper since it does not utilize the flatter periphery for its computation. A steeper Best-Fit shape will hide or mask conical areas. In the example of FIGURES 3A & 3B, the posterior elevation map appears to have a significant central island, but a closer inspection reveals a large area (diameter 9.00 mm) was used to compute the BFS.

When the map is recomputed with a smaller diameter (7.5 mm) the prominent anterior island disappears. This shows the importance of standardizing the area used to compute the BFS for both qualitative and quantitative analysis.

Because of the variable nature of the BFS and the need for the clinician to have normal values to screen patients, we no longer recommend the routine use of the AUTO setting.

The MAN (manual) setting is user selectable. The user may choose any size optical zone and the system will utilize all the data within that user defined optical zone to compute the Best-Fit-Shape. The value of this setting is that it allows a consistent area to be specified (see discussion in the following pages). The limitation is that the system will accept all the data points within that area whether the data is actual or extrapolated. The incorporation of extrapolated data, if excessive (poor quality scan), can lead to erroneous results. To allow for the generation of normal values and to allow for both patient comparisons over time and to compare different patients it is necessary to utilize the same BFS constructs. It is our recommendation that setting the system to MAN with a BFS diameter fixed at 8.0 mm gives the best results for the following reasons: